Improve Bash heredoc highlighting (#28185)

Release Notes:

  - Improved Bash heredoc highlighting

| Zed 0.180.2 | With this PR |
| --- | --- |
|
![Image](https://github.com/user-attachments/assets/aa2534af-53df-4f01-988e-f18ec52a2b62)
|
![Image](https://github.com/user-attachments/assets/8fc92113-41f2-4249-ab81-6beb0a1469ca)
|

```bash
cat << EOT >> hello.txt
hello world
EOT
```

- `<<`: `operator`
- `EOT`: `string`

crates/languages/src/bash/highlights.scm

@@ -3,6 +3,7 @@
   (raw_string)
   (heredoc_body)
   (heredoc_start)
+  (heredoc_end)
   (ansi_c_string)
   (word)
 ] @string
@@ -54,6 +55,7 @@
   "$"
   "&&"
   ">"
+  "<<"
   ">>"
   ">&"
   ">&-"